Sunbathers who bronze beautifully have natural selection to thank. Research published[2] in the Proceedings of the National Academy of Sciences suggests that the ability to tan is a trait that evolved several times in mid-latitude regions, such as China and the Mediterranean, where the sun's intensity varies dramatically from season to season. If inhabitants of these regions had consistently dark skin, which blocks the sun's rays, they wouldn't have produced enough vitamin D in the winter. If they had consistently light skin, their bodies would have been robbed of folate, a light-sensitive vitamin essential for cell division and repair. Folate is especially important during pregnancy; too little can result in birth defects. Indeed, the researchers posit that sun-induced folate deficiency, rather than skin cancer or sunburn, was the driving force behind the evolution of dark skin and tanning.